The Clay Mineralogy Of Fracture Coatings Observed In Mudrock Cuttings From The SAFOD Borehole

A number of mudrock drill cuttings have been investigated from shear zones at 1500 m and 3066 m depths showing abundant polished surfaces with occasional slip striations. Electron microscopy (SEM, TEM) and X-ray mineral identification study reveal the occurrence of thin clay film coatings containing smectite, chlorite, illite, quartz, albite and a possible serpentine mineral. This type of mineral assemblage was also observed along adjacent microveins and a freshly opened fracture. In order to compare these surface minerals with the drilling mud smectite, they were sequentially removed by ultrasonic treatment. The extreme sensitivity of this clay material under the TEM electron beam confirms the presence of abundant smectite on the polished mudrock surfaces, which shows compositional differences from that of the drilling mud. It contains notably more Fe, Mg and K and has a lower Si-Al ratio than the drilling mud montmorillonite. Additional differences between these two types of smectite could be recognized after hydration and chemical treatment (ethlylene glycol and glycerol solvation) whereby the natural smectite phase develops less hydrated states. In an attempt to examine the natural hydration states of these swelling smectite minerals, onsite impregnation (LR White resin) techniques have been applied. We suggest that these well oriented smectite coatings are partly precipitated from percolating solutions during fault creep.